An electroluminescent device utilizes the electroluminescence (EL) phenomenon, wherein an electron injected from a cathode and a hole injected from an anode form an exciton in the polymer emitting layer. A light of a particular wavelength is emitted from the formed exciton. The electroluminescent device does not need back light and has many advantageous features (e.g., light weight, thin thickness, self-illumination, low driving voltage, fast switching speed, etc.) so that it can be widely utilized as a new display device.
As reported by R. H. Friend in 1990, the polymer electroluminescent device in which the thin film is formed by spin coating is fabricated by a more simple process when compared to the low molecular organic electroluminescent device in which the thin film is formed by thermal vapor deposition. Furthermore, the device has a high thermal stability and an excellent mechanical strength. Therefore, many institutes have studied and investigated the polymer electroluminescent device, especially in order to improve the luminance efficiency thereof.
The studies for improving the luminance efficiency are primarily focused on a hole injecting layer or an electron injecting layer. For example, one investigation discloses that the hole injecting layer or the electron injecting layer is filled with an ionomer to improve the luminance efficiency (Lee H. M. et al., Appl. Phys. Lett., 72, 2382, 1998). However, the ionomer limits the free movement of the ion, whereby the injection of the electron or the hole becomes restricted and the durability thereof may become significantly deteriorated if the ion of the ionomer is diffused into the polymer emitting layer.
Moreover, it was described that lithium fluoride (LiF), which could effectively inject the electron, was used in the electron injecting layer to increase the luminance efficiency (T. M. Brown et al., Applied Physics Letters, 77(19), pp. 3096–3098, 2000). The material, however, requires a vacuum deposition so that it is not possible to apply the material for fabricating a large area of device.
In addition, U.S. Pat. Nos. 5,537,000, 5,817,431 and 5,994,835 suggested the methods of using an inorganic nanoparticle, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), chelate metal complex and the like for injecting the electron more effectively. However, it is difficult to fabricate a large area of the device by such method due to the vacuum deposition process.
Therefore, there is a need in the art for a polymer electroluminescent device, which has the improved luminance efficiency and which can be prepared in an easy and convenient manner.